Welded metal bellows of accordion shape having sealing performance and flexibility can be compressed until the bellows plates come into close contact, and are accordingly employed for sealing moving parts in various kinds of machines. For example, a welded metal bellows employed for sealing a gate valve capable of airtightly opening and closing an aperture of a vacuum treatment chamber employed in a fabrication step of a semiconductor fabrication device serves to seal off the vacuum side from atmospheric air side, and, as shown in FIG. 4, is formed into an accordion shape by welding together single bellows plates 50 at the inside diameter side and outside diameter side thereof, and is adapted to perform cyclic contracting action at high speed of several million strokes (hereinafter referred to as “Prior Art 1”). In the event that foreign matter present in the vacuum environment, for example, metal particles 51 or the like infiltrates the bellows plates 50 during this time as shown in FIG. 4 (a), owing to the “lever” principle, the bellows plates 50 experience bending deformation as shown by the phantom lines during contraction, and in the course of performing cyclic expansion and contraction, fractures arise in proximity to the metal particle 51 in the bellows plates 50, as shown in FIG. 4 (b), ultimately leading to failure. A resultant problem is that the bellows becomes unusable after an unexpectedly short service life.
Meanwhile, various welded metal bellows have been proposed to date; for example, a welded metal bellows 60 like that shown in FIG. 5 (hereinafter referred to as “Prior Art 2”; see Patent Document 1 for example) is known to have excellent corrosion resistance and operational spring performance. The bellows plates 61 constituting the accordion structure of the bellows 60 of Prior Art 2 are formed as double layers, by disposing austenitic stainless steel sheets 62 to the sealed fluid side, disposing spring steel sheets 63 to the non-sealed fluid side, and affixing them together by welding at the outside diameter side and inside diameter side.
A welded metal bellows 70 like that shown in FIG. 6 (hereinafter referred to as “Prior Art 3”; see Patent Document 2 for example) is known to offer a bellows able to withstand high pressure, without sacrificing the flexibility of the bellows. The bellows plates 71 constituting the accordion structure of the welded metal bellows 70 of Prior Art 3 are formed as double layers of two disks 72, 73 of uniform thickness, affixed by welding at the outside diameter side and inside diameter side. The disks 72 on the atmosphere side are furnished with vent holes 74 so that no gas pockets are present between the disks 72, 73.
However, Prior Art 2 shown in FIG. 5 is intended to offer a welded metal bellows of excellent corrosion resistance and operational spring performance, and takes no measures against cyclic bending deformation caused by infiltration of foreign matter. A resultant problem is that, in a case in which, for example, foreign matter 64 has infiltrated between two bellows plates 62, 62 on the sealed fluid side, the two bellows plates 62, 62 may be subjected to cyclic bending deformation, leading to successive or simultaneous failure of the two bellows plates 62, 62.
Moreover, Prior Art 3 shown in FIG. 6 is intended to offer a bellows able to withstand high pressure, without sacrificing flexibility; and because a vent hole 74 is formed in the bellows plate 72 on the atmosphere side, a resultant problem is that when failure occurs due to cyclic bending deformation of the bellows plate 73 on the opposite side from the atmosphere side caused by infiltration of foreign matter 75, sealing action can no longer be maintained, so in the end, the life is no better than in the case of a single bellows plate.